1. Field of the Invention
The invention relates to physiological signal monitoring systems, and more particularly to an interference filter for connection to or incorporation with a lead set supplying physiological signals to a patient monitor.
2. Description of the Prior Art
It is standard practice to monitor the heart (EKG) and other physiological signals of a patient during a medical procedure. Surgical procedures typically use an electrocautery device in which a surgical knife is supplied with a relatively high level of radio frequency (RF) current so that blood vessels and other tissues are cauterized and sealed immediately upon cutting. However the presence of such high level RF signals and energy on the body of the patient, and particularly in the region of the sensors used to acquire EKG signals, can cause severe electro-surgical interference (ESI) which can easily disrupt the operation of the patient monitor and cause a burning of the patient at the sensor site. High level RF interference signals can also come from other sources, such as the pulsed RF signals used during a magnetic resonance imaging procedure. Typically, if no precautions are taken, such high level RF signals will damage the sensitive input circuits of the patient monitor, and at the very least, so disrupt the input circuits as to cause them to overload and prevent monitoring of the physiological signals of the patient for many seconds. Such disruption in patient monitoring is highly undesirable.
Accordingly, the prior art has addressed the following issues during Patient monitor design:
1. The shielded lead wires which connect a plurality of patient mounted EKG electrodes to the patient monitor unwittingly provide a conduction path for these RF interference currents. If the RF current picked up by the electrodes is high enough, the skin may be burned at the electrode sites. The current path is from one electrode wire through its cable capacitance to its shield and back through the shield capacitance of another electrode cable to the electrode wire for that other electrode. (This assumes, as is typically the case, that the shields for all electrode wires are connected together at some point in the conduction path leading up to the signal processing circuits at the front end of the patient monitor.)
This potential hazard is typically prevented in the prior art by designing special electrode sets having high impedance elements such as inductors and/or resistors in series with the signal conductor connected to each electrode, such as at the contact point or "grabber" of the EKG electrode. U.S. Pat. No. 4,951,672 is typical thereof, and has series connected resistors in the lead wire sets for reducing the RF signals caused during magnetic resonance imaging. Such special electrode sets are typically only used in such limited and specialized situations because the high impedance elements may not be compatible with the monitoring of other physiological signals of the patient, such as respiration monitoring which applies a low level current to the patient via the EKG electrodes, and determines respiration using impedance techniques. On the other hand, U.S. Pat. No. 4,800,894 is representative of another prior art solution which simply avoids this problem completely, by using, for example, a "time out" circuit connected between the EKG electrode lead wire set and the signal input to the patient monitor. The time-out circuit literally removes all signal from the patient monitor signal inputs during the detected presence of RF interference.
2. If EKG signals of the patient are to be monitored during such procedures, the RF interference voltages at the patient mounted electrodes must be filtered. Typically, averaging filters are effective at removing such RF interference signals. Furthermore, such as shown by U.S. Pat. No. 4,245,649, the patient monitor input circuits which amplify the EKG signals typically use voltage clamps for protecting the input circuits from excessive voltages, such as those generated by a defibrillator. However, if the RF interference voltages at the electrodes is high enough, the action of the averaging filter will not provide a sufficient reduction in the level of the interference signal and the voltage clamps will conduct, causing rectification and subsequent translation of the RF signals to lower "in band" signal frequencies. Such frequency translation typically causes interference with proper monitoring of the EKG and other physiological signal.
A combination of filtering inside the patient monitor and the high impedance elements in the electrode lead wire or the lead wire grabbers have also been used to filter the RF interference voltage and to reduce the amplitude of the voltage below the clamping voltage of the amplifier protection circuits. However, effectively filtering such voltage to prevent the clamps from operating in the impedance respiration circuits is problematic because the respiration circuits may operate at a frequency which is close to that of the RF interference signal, e.g., the frequency of the electrosurgical RF signal. Furthermore, as noted above, the respiration circuit of the monitor typically can't tolerate a high impedance in series with the electrodes because the impedance changes experienced during . respiration monitoring are very low.
It would be desirable to provide a more effective and versatile RF interference filter arrangement for a patient monitoring system.